1. Field of the Invention
This invention relates to an improved method of and apparatus for the manufacture of stiff, strong, lightweight structures, and more particularly, to an improved method of and apparatus for the manufacturing of silicon carbide and/or silicon lightweight structures by the utilization of improved vapor deposition techniques. Further, this invention relates to a triangular chemical vapor deposition arrangement which can provide high yields of thick ceramic materials or parts from a vapor deposition system.
2. Description of Related Art
In the manufacture of ceramic materials by chemical vapor deposition, gases are introduced into a hot furnace where they react at the walls of the furnace or at the surface of a substrate or mandrel positioned in the furnace to form a solid deposit or coating thereon. Typically, a vacuum furnace designed in the shape of a tubular cylinder having a circular cross section is used for chemical vapor deposition. In the formation of a coating of a ceramic material such as silicon carbide (hereinafter referred to as "SIC"), methyltrichlorosilane (CH.sub.3 SiCl.sub.3, for convenience termed as "MTS" hereinafter), hydrogen (H.sub.2) and argon (Ar) gases are introduced in the reaction chamber through stainless steel injectors. Since MTS is a liquid at room temperature, Ar gas is bubbled through the MTS and carries MTS vapor to the injectors. Unreacted gases, products of reaction, and undeposited solids are evacuated from the furnace and cleaned in a gas scrubber. Thick deposits (greater than 10 mils) of SiC can be manufactured using this process. Typical conditions for the manufacture of SiC by chemical vapor deposition are:
______________________________________ Substrate Temperature 1350.degree. C. (2462.degree. F.) Furnace or Reaction Chamber Pressure 200 torr Partial Pressure of Gases Ar 125 Torr H.sub.2 60 Torr MTS 15 Torr ______________________________________
SiC parts are fabricated by the aforementioned process on a mandrel which is placed perpendicular to the flow, i.e., an impinging flow configuration. The reagents are introduced from several injectors which impinge on the mandrel at different locations and thus produce a more uniform deposit over the whole mandrel area. Efficient recovery of the deposited material without cracking or stressing is an important issue. In order to prevent deposited material from cracking, the mandrel may be isolated from the rest of the furnace using a gas shroud technique as disclosed in Keeley et al., U.S. Pat. No. 4,990,374; or a flexible body is used to prevent backside growth on the mandrel as disclosed in Goela et al., U.S. Pat. No. 4,963,393, both of which patents are assigned to the assignee of the present invention. The impinging flow configuration is preferred when specific parts are to be manufactured by deposition of material on male molds; such as, cones, discs, and cylinders of uniform thickness are required. However, when the objective is to fabricate large amounts of bulk sheet stock or deposit material in female molds, this configuration is inferior due to low values for reagent utilization efficiency.
Reagent utilization efficiency, with respect to vapor deposited material, in general, is defined as a ratio of the weight of material deposited on the mandrel to the total weight of material to be deposited that is contained in the reagents. With respect to SiC, reagent utilization efficiency is defined as a ratio of the weight of SiC deposited on the mandrel to the total weight of SiC in the reagents. In an impinging flow configuration, the reagent utilization efficiency is usually less than 20%. Since the walls of the chemical vapor deposition reactor are also heated, material may deposit on these walls and in the exhaust regions of the furnace. In most cases this material is treated as waste. In principle, one can minimize this waste by increasing the size of the mandrel. However, this requires increasing the furnace diameter, which is costly in a vacuum system. Preferably, the mandrel or mandrels should be so arranged in a chemical vapor deposition furnace to provide the maximum surface area available for deposition while occupying a minimum amount of the furnace floor surface area.
Further, undeposited solids must be removed by a filter system prior to the gases entering the vacuum pumping system. Due to the physical properties of SiC (hardness, 2540 kg/mm.sup.2 (Knoop 500 g load); fracture toughness, 3.3 Mn/m.sup.1.5 (micro-indention); and density 3.21 g/cm.sup.3), particles of undeposited material which pass through the exhaust system result in significant wear on process piping, seals, filters and other particulate removal components. Depositing SiC, which would normally be exhausted from the chemical vapor deposition furnace, would present the added advantage of reducing the wear on exhaust gas processing equipment. The resulting reduction in equipment cost associated with the manufacture of SiC would significantly reduce the overall cost of manufacture.
A deposition chamber in which the flow is parallel to the deposition surface provides good potential to obtain high deposition efficiency. Four-sided deposition chambers, formed from mandrel plates in the shape of a box which is open on both ends for the passage of reagents, are known in the art. Normally, sheets of material deposited on the inside surfaces of the mandrel plates arranged in this manner tend to bow and may crack. This condition would be exaggerated in the case of SiC due to its extreme hardness and elastic modulus. Further, material deposited on the inside surfaces of the four walls of the mandrel box grow together at the corners during the deposition process. Normally, this does not present a problem with soft material which is weaker than SiC, as it can be scored, fractured at the score line and easily machined. Due to the unique properties of SiC, extreme hardness and high strength, the scoring, fracturing and machining of SiC would be extremely difficult. Further, this additional step reduces the process yield.